Flat steel products are especially used in the field of automobile chassis construction, where particularly high demands are made on the formability and visual appearance of the components formed from such flat steel products.
Where reference is made here to flat steel products, these are rolled products such as steel strips or sheets, and blanks and sheet bars obtained therefrom.
Flat steel products intended for chassis construction or comparable applications are typically provided with a surface structure which features a defined roughness and a likewise defined peak distribution, in order to satisfy customer-specific demands that exist with regard to formability and surface impression (paintability and paint gloss). A typical example of corresponding specifications from the automotive industry sector is an arithmetic mean roughness (called “roughness” for short hereinafter) Ra of 1.1-1.6 μm with a peak count RPc of at least 60 l/cm. Roughness Ra and peak count RPc are determined according to Stahleisenprüfblatt [Steel and Iron Test Specification] SEP 1940 by means of a stylus instrument according to ISO 3274.
A further criterion for determination of the surface characteristics to be achieved for optimal paintability and optimal paint gloss is called the “waviness characteristic Wsa(1-5)”, called “Wsa” for short hereinafter, which is determined according to Stahl-Eisen-Prüfblatt SEP 1941:2012-05 after 5% plastic elongation by the Marciniak cup test. Typical requirements for Wsa values are from 0.35 μm to 0.40 μm. Particularly good paint gloss is established at Wsa values of ≤0.35 μm, especially <0.30 μm. In order to achieve such low Wsa values, peak counts RPc of at least 75 l/cm and roughnesses Ra of 0.9-1.4 μm are required.
In the production of cold-rolled flat steel products, the material characteristics Ra and RPc are typically established by temper rolling after the recrystallization annealing, through which the flat steel products pass after the cold rolling in order to assure optimal formability thereof.
“Temper rolling” is understood here to mean partial rolling or further rolling performed after the recrystallization annealing, in which the flat steel product is subjected to a low deformation of about 0.2%-2.0%, which is referred to here as “temper reduction”. The temper reduction is determined here by a comparison of the peripheral speeds of the deflecting rollers that are provided with position-determining devices, upstream and downstream of the roll stand in which the flat steel product is temper-rolled. The temper reduction D arises from the difference in distance traveled by the deflecting rollers (distance traveled at inlet s1, distance traveled at outlet s2), calculated as D=[(s2−s1)/s1]*100.
The combined requirement for “high peak count RPc” and “high roughness Ra” is fundamentally a complex manufacturing task. This is because a high roll roughness required for achievement of high Ra values fundamentally entails a low peak count RPc, since the increasing surface fissuring (=roughness) of the roll increases the distance from wave crest to wave crest on the roll surface and hence reduces the number of peaks that can be formed on the flat steel product. A further complicating factor is that, even in the case of dry temper rolling, a peak transfer loss of about 20% is recorded in the transfer of the peaks present on the roll surface to the flat steel product being rolled in the particular case.
An additional factor is the rule that, if the temper reduction D chosen is too high, the roughness Ra will be too high. If, by contrast, the temper reduction D is set too low, there could be untempered strip edges, especially in the case of broad strip dimensions. At those points, the Ra and RPc values are then too low.
The temper reduction D also cannot be varied as desired with regard to the mechanical properties of the steel substrate. Too low a temper reduction D only inadequately counteracts a marked yield strength. Too high a temper reduction D, by contrast, can cause the strength of the steel substrate to be too high in a non-correctable manner because of excessively intense cold solidification.
The softer, broader and thinner the flat steel product to be produced is, the greater the demands on the temper rolling. A “soft” steel is understood here to mean a steel which, in the recrystallized state and after the temper rolling, has a yield strength Rp0.2 of not more than 180 N/mm2 and a tensile strength Rm of not more than 340 N/mm2. The result of this in practice is that flat steel products of the type in question with automobile-typical dimensions can currently only be produced with the desired operational reliability with a great deal of complexity. Particularly critical steels are those having a yield strength Rp0.2 of max. 150 MPa and a tensile strength Rm of not more than 310 MPa.
There are various known proposals for making this degree of complexity controllable in practice, and for producing flat steel products which are to provide optimal prerequisites for painting with a gloss that meets even the strictest requirements.
One example of this is the method, known from EP 0 234 698 B1, of producing a steel sheet suitable for painting. This method envisages producing a regular pattern of depressions in the surface of a temper roll by means of a beam of energy. The flat steel product to be processed is temper-rolled by means of two working rolls, at least one of which has been processed in the manner specified above. The reduction in cross section achieved via the temper rolling is to be not less than 0.3%, in order to transfer the pattern from the working roll to the surface of the steel sheet. In this way, a steel sheet having an average surface roughness Ra within the range from 0.3 to 3.0 μm and a microscopic form that forms the surface roughness, consisting of trapezoidal elevation regions with a planar upper surface, groove-like depression regions formed in such a way that they completely or partly surround an elevation region, and planar middle regions formed between the elevation regions outside the depression regions such that they are higher than the base of the depression regions and lower than or of the same height as the upper surfaces of the elevation regions, is to be obtained. At the same time, the elevations and depressions are to have particular geometric dependences on parameters including the diameter of the depressions formed into the working temper roll.
A comparable proposal has been made in DE 36 86 816 T2. This too has proposed introducing a homogeneous surface roughness pattern into the surface of a cold-rolled flat steel product, which leads to a surface roughness Ra of 0.3-2.0 μm.
Finally, WO 2011/162135 A1 discloses a thin cold-rolled steel sheet and a method of production thereof. This steel sheet consists of a steel having, in % by weight, 0.10% or less of C, 0.05% or less of Si, 0.1%-1.0% Mn, 0.05% or less of P, 0.02% or less of S, 0.02%-0.10% Al and less than 0.005% N, the remainder consisting of Fe and unavoidable impurities. The steel sheet having these characteristics is subjected to an annealing treatment in which it is annealed at an annealing temperature of 730-850° C. for at least 30 s and then cooled to a temperature of not more than 600° C. at a cooling rate of at least 5° C./s. The annealed cold-rolled flat steel product obtained thereafter has a microstructure consisting mainly of ferrite, having an average crystal grain diameter of 5-30 μm. Finally, the flat steel product is temper-rolled using a roll having a surface roughness Ra of not more than 2 μm. The stretching ratio achieved via the temper rolling is set as a function of the average crystal grain diameter of the thin cold-rolled annealed sheet.